AC to DC electrical power rectifiers are used in a wide variety of applications, including power supplies for various types of electrical equipment and in motor drives. The simplest and least expensive type of rectifier uses a full or half bridge of diodes to rectify single-phase or three-phase AC input power to DC power. While rectifiers formed of passive diodes are inexpensive and relatively reliable, such rectifiers can introduce significant harmonic distortion to the AC power system to which the rectifiers are connected. The total harmonic distortion (THD) introduced by such diode rectifiers may not satisfy current standards and regulations such as the IEEE 519 standard. To address these limitations, rectifiers have been developed that include active switching devices, such as IGBT transistors, connected in parallel with the diodes of the bridge (e.g., six switches for a full-bridge three-phase AC to DC rectifier), with the switches being controlled to operate to provide close to unity power factor and reduced THD as compared to passive rectifier bridges. Such active rectifiers also permit bidirectional power flow, allowing power from regenerative loads, such as large motors, to be delivered back through the rectifier to the AC power system. A disadvantage of such active rectifiers is that the switching devices are much more expensive than passive diodes, particularly since each active switch must be capable of handling the full rated current and voltage of the system.
In addition to full-bridge active rectifiers, half-controlled rectifiers have also been developed in which half of the rectifier bridge is formed of passive diodes and the other half has active switching devices in parallel with the diodes. For a three-phase AC to DC rectifier, such bridges can be formed of a bridge of six diodes and three active switches. Half-controlled three-phase pulse width modulated boost rectifiers thus can potentially be lower in cost as compared to a rectifier having a full bridge of active devices. See, C. H. Treviso, et al., “A Three-Phase PWM Boost Rectifier with High Power Factor Operation and an Acceptable Current THD Using Only Three Switches,” Proc. of E.P.E '97, 1997, pp. 2.934–2.939; J. Kikuchi, et al., “Performance Improvement of Half-Controlled Three-Phase PWM Boost Rectifier,” Proc. of IEEE Power Electronics Specialists Conference, 1999, Vol. 1, January 1999, pp. 319–324. Such circuits have also been developed for the purpose of supplying multiple isolated DC buses and multiple loads by employing two complementary half-controlled circuits. J. Kikuchi, et al., “Complementary Half-Controlled Three-Phase PWM Boost Rectifier for Multi-DC-Link Application,” Proc. of IEEE Applied Power Electronics Conference, 2000, Vol. 1, January 2000, pp. 494–500. For an individual half-controlled circuit, the cost of the system inherently will be reduced since only three active devices are used as compared to six devices for the fully controlled counterpart. However, the price to be paid is higher THD and the presence of low order even harmonics on both the AC side and the DC side. Thus, both the system efficiency and the performance are degraded as compared to the full active bridge. The overall THD of the system was found to be improved to some extent by using a lagging power factor current command in J. Kikuchi, et al, 1999, supra.
However, appreciable THD still remained which was far from satisfactory given the IEEE 519 standards. In addition, a lagging power factor command may not suit all rectifier loads and, more typically, a unity power factor interface is preferred or required. Some of the problems affecting half-controlled rectifiers, such as the lower order harmonics from the AC side and on the DC bus, are partially solved by combining two complementary half-controlled rectifiers and applying a coordinated central control algorithm, as discussed in J. Kikuchi, et al., 2000, supra. By using this approach, the AC side THD was improved markedly, but the DC side lower order even harmonics still existed. The major objective was to have an isolated DC bus for each half-controlled circuit.
In boost rectifiers of the type discussed above, inductors are normally used in series with the input phases in order to reduce the amplitude of the switching frequency harmonics of the system. Because the phase currents will contain appreciable lower order harmonics in a conventional three-phase half-controlled rectifier circuit as compared to a fully controlled rectifier circuit, the inductors will incur extra losses. In addition, such systems may be prone to acoustic noise problems.